While 778 exoplanet discoveries have been confirmed as of June 15, with increasingly good data about their masses and orbits, direct measurement of their chemical composition continues to be difficult. The reasons for that are obvious: exoplanets (planets orbiting other stars) are small compared to the vastness of space, and they emit little light of their own. Therefore, determining whether a given exoplanet has an atmosphere is challenging, much less what gases may be found in it.

However, new observations of τ Boötis b (pronounced approximately "tau boo-OOTis") have revealed a way to identify at least some of the components in an atmosphere. Matteo Brogi et al. used the Very Large Telescope (VLT) in Chile to measure the absorption of light by carbon monoxide (CO) molecules in the atmosphere. Additionally, they used their high-resolution data to refine the orbital data for the planet. While τ Boötis b is relatively close to Earth and orbits very close to its host star—meaning it's easier to study than most other exoplanetary systems—these results show in principle that it is possible to directly measure the atmospheric absorption of light. This method potentially improves the exoplanet-hunter's toolkit significantly, allowing more precise orbital characterization, even when the planet doesn't eclipse its host star.

τ Boötis b is one of the first known exoplanets, first observed in 1996 orbiting the yellow-white (F-class) star τ Boötis A. The planet's mass is about 6 times that of Jupiter, and it orbits at a distance of about 1/10 that of Mercury. Therefore, it is a "hot Jupiter," one of a number of giant planets orbiting very close to their host stars. The star is only about 51 light years from Earth, meaning it is easier to observe than many other exoplanetary systems.

Astronomers have identified the chemistry of other exoplanets, but those were measured during transits: when the planet passes between its host star and Earth, blocking a small amount of light. Most notably, observations of HD 209458 b revealed sodium, water, hydrogen, and metallic oxides (titanium and vanadium) in a stream from the planet. In contrast, τ Boötis b is not a transiting exoplanet, but it orbits even more closely than HD 209458 b. That means it has a higher cloudtop temperature (the temperature at the edge of the atmosphere), which makes it relatively bright in infrared light on the daylight side of the planet.

To measure the spectrum of τ Boötis b, the researchers used the Cryogenic Infrared Echelle Spectrograph (CRIRES) on one telescope in the VLT array. As the name of the instrument suggests, these observations were in the infrared portion of the electromagnetic spectrum, which provides the best hope for separating the signature of the planet from its host star. The observers specifically looked for CO absorption—bombardment by ultraviolet radiation from the star was expected to produce this molecule in the atmosphere of a planet in such a close orbit.

They found a strong CO absorption line, justifying their choice of observing target. However, they also were able to measure the motion of τ Boötis b by tracking the CO absorption signature over time as the planet orbited. While this technique is established for binary star systems, this is the first time such a measurement has been performed for an exoplanet. The researchers used the speed of motion of the planet to refine the data on τ Boötis b, revealing it to be more massive than previously thought and finding its inclination (how tilted its orbit is relative to our line of sight). They measured the mass of τ Boötis b to be between 5.67 and 6.23 times the mass of Jupiter; they also found the orbit is nearly circular and tilted about 45° towards Earth.

The advances provided by these results are twofold: the researchers determined that it is possible to measure atmospheric absorption even in non-transiting exoplanets, and showed that the chemical signature in turn can be used to determine the mass and orbital characteristics of the planet. While τ Boötis b is a relatively easy exoplanet to study—it is massive, close to its host star, and proximate to Earth—the methods should be extendable to other exoplanetary systems. The authors even hinted that atmospheric studies such as this could measure the rotation of the planet, something that has not yet been done. Similarly, while the present study focused on CO, there is no inherent reason future observations couldn't look for water or other molecules—including the chemical signatures of life.

same here. i wonder if this is how they felt in the 50s when man was first venturing beyond our own atmosphere...

the whole pluto debacle made me wonder about those plates on the sides of the V'ger probes... especially now that we've discovered other pluto-like objects at the same general distance. do we send out revised plates?

the whole pluto debacle made me wonder about those plates on the sides of the V'ger probes... especially now that we've discovered other pluto-like objects at the same general distance. do we send out revised plates?

This is some bad-ass-tronomy! They correlate the known planet velocity from its known orbit with spectroscopic variation and can tease out what is light and its absorption from the planet (vs say star emission/absorption or Earth atmosphere absorption).

And they could use just a few days of closely spaced observation.

However they say, worryingly, that:

"This can be achieved without destroying the planetary signature only because the signal from the planet moves significantly in wavelength during our observations, due to its large change in radial velocity along the orbit."

So difficult or impossible for Earth analogs, as far as the quick check goes.

Reversing this, looking at wavelengths like CO and ozone (photoreactions) or CO2 and water (greenhouse gases) that gives large absorption, they should in principle be able to use this as detection method where neither transits nor wobbles may work. Using two wavelengths should still enable making a bona fide detection with two independent means of observation in some cases.

So, when are we going to see our first inhabited planet? They promised us habitables "within 2-3 years" some time after Kepler was launched and assessed, and they delivered (habitables around M stars in small orbits first).

Now we are supposed to wait 1-2 decades for seeing oxygen on a habitable planet by projecting trends!? But this shows technology leapfrogs in fruitful areas. I want our first inhabited exoplanet yesterday already!